1. Field of the Invention
The present invention relates to an oil pressure control system for an automatic transmission system for use with automobiles.
2. Description of the Prior Art
In an automatic transmission system which includes a fluid torque converter, a transmission gear and friction engaging means for establishing a selected transmission engagement in said transmission gear, said automatic transmission system being controlled by an oil pressure control system said friction engaging means are engaged in various combinations by changing over of speed shift valves incorporated in said oil pressure control system in order to accomplish the most favorable transmission of power by said transmission gear according to the operational condition of the vehicle. The abovementioned changing over in the oil pressure control system is principally effected by a balance between a governor pressure which increases according to the increase of the vehicle speed and a counter acting force which is exerted by a spring force and/or a throttle pressure or other control pressures, said throttle pressure increasing according to increase of the engine torque, or particularly, of the throttle opening. In this case, if the same balancing condition is used for shifting said speed shift valves from a lower speed shift position (gear) to a higher speed shift position (gear) (upshifting), and for shifting same from a higher speed shift position (gear) to a lower speed shift position (gear) (downshifting), said upshifting and downshifting frequently occur when the vehicle is operated in the vicinity of the shifting point, thereby causing instability in operation of the automatic transmission system.